The invention described herein relates to the field of acoustic energy sources used in geophysical exploration and other marine operations. More particularly, the invention relates to an improved acoustic energy source system operable to generate low frequency seismic source energy in water.
Marine seismic vessels tow vibrators and discharge air guns, explosives and other acoustic projectors to generate seismic source energy in marine geophysical operations. The seismic source energy comprises a pressure pulse which travels downwardly through the water and underlying subsurface geologic structures. The source energy is partially reflected from interfaces between the geologic structures and is detected with geophone or hydrophone sensors.
Conventional source signals for marine geophysical operations are generated with acoustic sources such as compressed air guns. U.S. Pat. No. 4,180,139 to Walker (1979), U.S. Pat. No. 4,285,415 to Paitson (1981), and U.S. Pat. No. 5,228,010 to Harrison (1993) disclosed different mechanisms for discharging compressed air into water to generate acoustic source energy.
U.S. Pat. No. 3,896,889 to Bouyoucos (1975) disclosed a mass oscillation system for generating acoustic source energy in water. Other devices generate an acoustic signal by transmitting high velocity water jets in the underwater water environment. U.S. Pat. No. 4,131,178 to Buoyoucos (1978) and U.S. Pat. No. 4,153,135 to Bouyoucos (1975) disclosed a moveable piston for generating high velocity water jets.
Other acoustic energy sources have been developed for sonar and other vessel detection systems. U.S. Pat. No. 4,651,044 to Kompanek (1987) disclosed a sonar transducer formed having a plurality of sectionalized piezoelectric elements attached to the interior wall of a slotted cylinder. Each element was constructed with a ceramic material having piezoelectric characteristics and was bonded with adhesive to the interior cylinder wall.
Slotted cylinder transducers require the slot to permit expansion and contraction of the cylinder as the transducer elements vibrate the cylinder. For underwater applications, a flexible watertight skin covers the slot as the cylinder expands and contracts. In the absence of a pressure compensation system, the hydrodynamic pressure of the water can compress the cylinder and cause the slot to close. The transducer is inoperable when this occurs.
Other variations and improvements to acoustic energy sources have been developed. U.S. Pat. No. 5,122,992 to Kompanek (1992) disclosed a transducer member having a closure member extending in a U-shaped configuration, and U.S. Pat. No. 5,267,223 to Flanagan et al. (1993) disclosed a compliant cover bonded to a transducer shell. Another application of slotted cylinders was disclosed in U.S. Pat. No. 5,592,359 to Kompanek (1997), wherein two slotted cylinders were fastened together for the purpose of separating oil from sediment.
Conventional seismic sources require a number of air guns to produce an acceptable source level in the desired frequency band. The large number of constituent air guns, connecting hardware, and air hose bundle supplying the air guns adds significant weight and frictional drag to towed arrays. Slotted cylinder and other conventional acoustic energy sources for underwater vessel detection operate at frequencies and power ranges unsuitable for geophysical operation. Accordingly, there is a need for an improved source generator for use in marine operations. The generator should be adaptable to different source energy requirements and should be easy to manufacture and deploy.